Support using "auto" when using margin shorthand

src/__tests__/boxModel.js

@@ -84,6 +84,39 @@ it('transforms shorthand and overrides previous values', () => {
   })
 })
 
+it('transforms margin shorthand with auto', () => {
+  expect(transformCss([['margin', 'auto']])).toEqual({
+    marginTop: 'auto',
+    marginRight: 'auto',
+    marginBottom: 'auto',
+    marginLeft: 'auto',
+  })
+  expect(transformCss([['margin', '0 auto']])).toEqual({
+    marginTop: 0,
+    marginRight: 'auto',
+    marginBottom: 0,
+    marginLeft: 'auto',
+  })
+  expect(transformCss([['margin', 'auto 0']])).toEqual({
+    marginTop: 'auto',
+    marginRight: 0,
+    marginBottom: 'auto',
+    marginLeft: 0,
+  })
+  expect(transformCss([['margin', '2px 3px auto']])).toEqual({
+    marginTop: 2,
+    marginRight: 3,
+    marginBottom: 'auto',
+    marginLeft: 3,
+  })
+  expect(transformCss([['margin', '10px auto 4px']])).toEqual({
+    marginTop: 10,
+    marginRight: 'auto',
+    marginBottom: 4,
+    marginLeft: 'auto',
+  })
+})
+
 it('transforms border width', () => {
   expect(transformCss([['border-width', '1px 2px 3px 4px']])).toEqual({
     borderTopWidth: 1,

src/transforms/index.js

@@ -9,7 +9,7 @@ import textDecorationLine from './textDecorationLine'
 import transform from './transform'
 import { directionFactory, anyOrderFactory, shadowOffsetFactory } from './util'
 
-const { IDENT, WORD, COLOR } = tokens
+const { IDENT, WORD, COLOR, LENGTH, PERCENT, AUTO } = tokens
 
 const background = tokenStream => ({
   $merge: { backgroundColor: tokenStream.expect(COLOR) },
@@ -39,7 +39,10 @@ const borderRadius = directionFactory({
   suffix: 'Radius',
 })
 const borderWidth = directionFactory({ prefix: 'border', suffix: 'Width' })
-const margin = directionFactory({ prefix: 'margin' })
+const margin = directionFactory({
+  types: [LENGTH, PERCENT, AUTO],
+  prefix: 'margin',
+})
 const padding = directionFactory({ prefix: 'padding' })
 const flexFlow = anyOrderFactory({
   flexWrap: {